The well-decomposed, relatively stable part of the organic matter found in aerobic soils. A complex aggregate of amorphous substances, formed during the microbial decomposition or alteration of plant and animal residues and products synthesized by soil organisms; principal constituents are derivatives of lignins, proteins and cellulose; humus has a high capacity for base exchange (CEC), combining with inorganic soil constituents, and for water absorption; finished compost may be designated by the general term humus.